1. Field of the Invention
The present invention generally relates to dynamic image sensors used in digital image acquisition devices such as cameras.
The present invention relates to such image acquisition devices, whether they are autonomous or part of a device comprising other functions such as, for example, a cell phone or an automobile vehicle.
2. Discussion of the Related Art
FIG. 1 very schematically shows, in the form of blocks, an example of an image acquisition device of the type to which the present invention applies. This device comprises an image sensor formed of photodiode cells and its control systems 10 (PIXEL ARRAY AND DRIVERS) providing the levels representative of the illumination of the different photodiodes. The image acquisition device also comprises a unit 11 for controlling sensor 10, comprised, among others, of an analog-to-digital signal converter 12 (ADC) and of a control and processing electronic circuit 14 (SE). The illumination levels are then exploited by a storage system 16 (MEM).
FIG. 2 shows an example of an equivalent electric diagram of a photodiode cell of an image sensor. The cell comprises a photodiode PD used in reverse mode and having its junction capacitance discharged by the photocurrent according to the received light intensity. The anode of photodiode PD is connected to a reference voltage (for example, ground M) and its cathode is connected to a node Q defining a reading point of the cell. Node Q is connected, by a switch RST, to a terminal 20 for providing a voltage VRST, which is positive with respect to reference voltage M. Voltage VRST enables to reset the cell (precharge the junction capacitance of the photodiode) between two successive image acquisitions. Node Q is further connected by a switch SEL to a device MES for measuring the discharge voltage of photodiode PD. The output of the measurement device is connected to a terminal V which provides a voltage transmitted to block 12.
In a simple so-called static sensor, the acquisition is performed during a fixed integration period. If the light intensity fully discharges a photodiode before the end of the integration period, the cell is saturated. The sensor is no longer capable of distinguishing between the highest brightness levels or ranges of the cells.
In a so-called highly dynamic sensor, this saturation is desired to be avoided by the dividing of the integration period into time intervals between which read node Q is recharged.
FIG. 3 illustrates, in a timing diagram, the operation of a dynamic image sensor. This timing diagram shows four courses of voltage V originating from a photodiode for four different luminosities during an integrated period TON.
This timing diagram shows that for a 5-lux illumination, the final level read at the end of the integration periods is not altered by the successive resets. The 50-lux voltage is altered by the first reset, but not by the next two following ones. The 250-lux level is altered by the first and second resets, but not by the third one. The 500-lux level is altered for all resets. For this high brightness level, the cell saturates despite the performed resets.
In this example, the integration period is divided into 4 intervals TINT1, TINT2, TINT3, and TINT4 between which the sensor is reset to successive decreasing voltage levels VRST1, VRST2, VRST3, and VRST4.
At the beginning of period TINT1, all photodiodes are precharged to voltage VRST1. The photodiodes are then discharged during the entire period TINT1. This first period avoids the saturation, but this saturation would be reached for period TON for certain brightness levels.
At the beginning of periods TINT2 and TINT3, all photodiodes are precharged to voltage VRST2, respectively VRST3. As for the first period, the saturation is avoided. The last period TINT4 does not avoid the saturation of higher levels. But photodiodes exposed to a high brightness, for example, 500 lux, are saturated and the data relative to the distribution of the saturated photodiode levels are lost.
In a usual dynamic sensor, the number of integration periods is adapted to avoid this saturation phenomenon. Conversely, if the image becomes darker, the number of intervals is decreased to improve the contrast of dark images.
Ideally, a homogeneous distribution of the cells in the entire excursion of the signal provided by the sensor (stored values or voltage levels) guarantees a good contrast. In practice, such a distribution is not achieved in a usual dynamic sensor. This phenomenon is illustrated by the right-hand portion of FIG. 3, which shows the cell distribution by brightness levels, assuming a sensor of 4xQ cells. It should be noted that quantities Q are not regularly distributed with respect to the brightness levels. Quantity αxQ represents the number of saturated cells.
Known examples of image sensors and their operation are described in U.S. Pat. Nos. 6,600,471 and 6,348,681.